Development of Cu-based oxygen carriers for chemical-looping combustion

In a chemical-looping combustion (CLC) process, gas (natural gas, syngas, etc.) is burnt in two reactors. In the first one, a metallic oxide that is used as oxygen source is reduced by the feeding gas to a lower oxidation state, being CO₂ and steam the reaction products. In the second reactor, the reduced solid is regenerated with air to the fresh oxide, and the process can be repeated for many successive cycles. CO₂ can be easily recovered from the outlet gas coming from the first reactor by simple steam condensation. Consequently, CLC is a clean process for the combustion of carbon containing fuels preventing the CO₂ emissions to the atmosphere. The main drawback of the overall process is that the carriers are subjected to strong chemical and thermal stresses in every cycle and the performance and mechanical strength can decay down to unacceptable levels after enough number of cycles in use.In this paper the behaviour of CuO as an oxygen carrier for a CLC process has been analysed in a thermogravimetric analyser. The effects of carrier composition and preparation method used have been investigated to develop Cu-based carriers exhibiting high reduction and oxidation rates without substantial changes in the chemical, structural and mechanical properties for a high number of oxidation-reduction cycles. It has been observed that the carriers prepared by mechanical mixing or by coprecipitation showed an excellent chemical stability in multicycle tests in thermobalance, however, the mechanical properties of these carriers were highly degraded to unacceptable levels. On the other hand, the carriers prepared by impregnation exhibited excellent chemical stability without substantial decay of the mechanical strength in multicycle testing. These results suggest that copper based carriers prepared by impregnation are good candidates for CLC process.